Compounds which can reverse-imagewise release a photographically useful group, or positive-working compounds, have been energetically studied, because they can serve various functions, which have never been performed by conventional precursors in a silver halide photographic material.
As for the positive-working compounds, immobile compounds disclosed in U.S. Pat. Nos. 4,199,354 and 3,980,479 have been proposed.
Those compounds can release photographic reagents in the reduced condition by undergoing an intramolecular nucleophilic reaction in the presence of an alkali, whereas when oxidized through a redox reaction in the light-sensitive material they have a decreased release rate of the photographic reagent. By utilization of this property, imagewise release of photographically useful groups from such compounds is feasible. However, they have certain disadvantages, e.g., generation of fog and deterioration of discrimination, which result from a timing lag upon competition between the oxidation and the alkaline hydrolysis, insufficient stability of the compounds themselves, and so on.
For the purpose of solving the problems of the above-described photographic reagent-releasing compounds of positive-working type, a great number of positive-working compounds have been developed on the basis of the idea that positive-working compounds themselves are converted to the oxidized form, from which photographically useful groups are released through the redox reaction with reducing agents.
Further, positive-working compounds which can release photographic reagents by an intramolecular nucleophilic substitution reaction after they are reduced, include those disclosed in U.S. Pat. Nos. 4,139,389, 4,139,379 and 4,564,577, and Japanese Patent Application (OPI) Nos. 185333/84 and 84453/82 (the term "OPI" as used herein means an "unexamined published Japanese patent application"); and positive-working compounds which can split off photographic reagents by an intramolecular electron transfer reaction after they are reduced, include those disclosed in U.S Pat. No. 4,232,107, Japanese Patent Application (OPI) No. 101649/84, Research Disclosure IV, No. 24025 (1984), and Japanese Patent Application (OPI) No. 88257/86.
Furthermore, positive-working compounds which can release photographic reagents by bond cleavage by reduction have been studied.
Examples include compounds which utilize the reductive cleavage of an N--S bond, as disclosed in West German Patent No. 3,008,588; and compounds which utilize N--N bond cleavage, disclosed in U.S. Patent No. 4,619,884. In addition, examples include .alpha.-nitro compounds which can release photographic reagents through acceptance of an electron and the subsequent cleavage of the single bond between a carbon atom and a hereto atom, as disclosed in West German Pat. No. 8,207,583; compounds which utilize the reductive cleavage of a single bond between a carbon atom and a hereto atom, for example, gemminate dinitro compounds which undergo .beta.-elimination of photographic reagents after the reductive cleavage of a bond between a carbon atom and a nitrogen atom (nitro group), which are disclosed in U.S. Pat. No. 4,609,610; and nitrobenzyl compounds disclosed in U.S. Pat. No. 4,343,893, which utilize the reductive cleavage of a single bond between a carbon atom and a hereto atom.
In recent years, European Pat. No. 220,746 A2 and Kokai Giho No. 87-6199 (May 20, 1987) have developed positive-working compounds satisfying two contradictory requirements of excellent stability, and higher activity upon processing, to permit more freedom of design and latitude in making photographic elements and operating processes.
Each of the compounds having functions as described above has many advantages. However, they are crystalline low-lomecular compounds, so emulsions containing them are unstable and precipitate crystals, and they can easily migrate among layers to exert undesirable effects upon other layers through diffusion.
Further, when a high-boiling organic solvent is used at the time of emulsification for incorporation of such compounds into a silver halide light-sensitive material, it frequently causes the softening of the layer in which it was contained to decrease the film strength and deteriorate adhesiveness between layers.